1. Field of the Invention
The present invention relates to a lens, mirror, prism, or similar plastic molded article or product produced by injection molding and included in an optical device, e.g., a copier, laser printer, facsimile apparatus, or similar image forming apparatus, and a method and apparatus for producing the same. More particularly, the present invention is concerned with a plastic molded article or product having, e.g., mirror surfaces and a fine undulation pattern transferred thereto with high accuracy by injection molding, and a method and apparatus for producing the same.
2. Discussion of Background
For injection molding, it is a common practice to use a mold assembly including an inner mold surface forming a cavity having a preselected volume, a transfer surface formed on the inner mold surface for transferring a mirror surface to a molded article or product, and a gate or sprue open at the inner mold surface and having a preselected opening area. Molten resin is injected into the cavity via the gate or sprue and then cooled. The resulting molded article or product is taken out by opening the mold assembly. While such a molded article or product, particularly a mirror, lens, prism, or similar optical element, is required to have an accurate mirror surface and a uniform refractive index, the mirror surface needing a high surface accuracy is caused to sink because the molten resin contracts at the time of solidification.
Injecting molding methods for solving the above-described problem are taught in, e.g., Japanese Patent Laid-Open Publication Nos. 3-128218, 8-234005, 3-151218, and 3-281213 (hereinafter xe2x80x9cPrior Art 1xe2x80x9d). In Prior Art 1, either a non-transfer surface, or a mold surface which faces a transfer surface, is formed with, e.g., a roughened mirror surface or a surface treated so as to have a lower wettability, or else use is made of a porous material. Injection is stopped just before a cavity is filled up with molten resin. Then, the molten resin is solidified by cooling without any dwelling. As a result, the roughened surface is caused to sink due to a difference in adhering force between the molten resin, the transfer surface, and the roughened surface. This prevents the mirror surface of the molded article or product from sinking. Alternatively, an overflow portion for receiving excess molten resin is located outside of the cavity. When the overflow portion begins to be filled, injection is stopped. Then, the molten resin is solidified by cooling without any dwelling. This also allows the roughened surface to sink due to a difference in adhering force between the resin, the transfer surface, and the roughened surface.
An injection molding method disclosed in Japanese Patent Laid-Open Publication No. 2-175115 (hereinafter xe2x80x9cPrior Art 2xe2x80x9d) injects molten resin into a cavity in which a porous member, which communicates with a compressed gas such as air, is provided on an inner mold surface so as to contact the non-transfer surface of a molded article or product. While dwelling and cooling are under way after the injection of the molten resin, air is fed to the non-transfer surface of the molded article or product via the porous member. With this method, a side of a cylindrical thin lens (i.e., the molded article or product) may be caused to sink.
Japanese Patent Laid-Open Publication No. 6-304973 (hereinafter xe2x80x9cPrior Art 3xe2x80x9d) proposes an injection molding method in which a non-transfer surface communicates with the outside air via a vent hole. During an interval between the beginning and the end of the injection of molten resin into a cavity, a pressure difference is generated between the transfer surface and the non-transfer surface of the resin. As a result, the non-transfer surface of the resin is caused to sink. Specifically, air is brought into contact with the molten resin, on a surface other than the mirror surface of the molded article or product transferred from the transfer surface of the mold assembly, via the vent hole and a bore communicating therewith, so that the cooling speed of the molten resin is lowered. At the same time, a preselected air pressure is fed to the vent hole in order to generate a preselected pressure difference between the mirror surface of the molded article or product and the vent hole. This allows only the portion of the resin facing the vent hole to sink, i.e., prevents the mirror surface of the molded article or product from sinking. In addition, because only the portion of the resin facing the vent hole sinks, a molded article or product can be produced by simple control over the amount of the molten resin to be injected into the cavity and without any strain being generated in the resin. The resulting molded article or product is therefore free from an internal strain and is provided with an accurate mirror surface.
Prior Art 3 further teaches that the vent hole may communicate with a compressor so as to apply a preselected air pressure to the portion of the resin facing the vent hole. With this configuration, it is possible to generate any desired pressure difference between the mirror surface of the molded article or product and the portion of the resin facing the vent hole, thereby causing the portion of the resin facing the vent hole to sink. In addition, the pressure difference is readily adjustable in order to further enhance the accuracy of the mirror surface without any internal strain.
Japanese Patent Laid-Open Publication No. 6-315961 (hereinafter xe2x80x9cPrior Art 4xe2x80x9d) teaches an injection molding method causing the non-transfer surface of resin to sink. In accordance with this method, the transfer surface of a mold assembly is heated to and held at a high temperature. The transfer surface side of the resin is heated to a high temperature until the injection of molten resin into a cavity ends.
However, the above-described injection molding methods of Prior Art 1, Prior Art 2, Prior Art 3, and Prior Art 4 have some drawbacks. More particularly, Prior Art 1, which relies on any one of the roughened surface, surface treatment, or porous material, results in an expensive mold assembly. Moreover, stopping the injection just before the cavity is filled up with the molten resin is extremely difficult. Should the correct timing for stopping the injection of molten resin not be realized, the relationship in the adhering forces between the transfer surface and the roughened surface would be inverted and would thereby cause the mirror surface of the molded article or product to sink or else result in a shortage of resin. In addition, because sinking cannot be provided with directionality and because setting the molding conditions is difficult, the configuration of the molded article or product is critically limited. It is more preferable that the injection of the molten resin be stoppable at any time within a broader range of times. However, in this case, the overflow portion, which is formed integrally with the molded article or product, must be removed by an extra step. Moreover, if the area of the opening of the gate or sprue for feeding the molten resin to the overflow portion is excessively small, the relationship in adhering force between the transfer surface and the roughened surface would also be inverted and would thereby cause the mirror surface of the molded article or product to sink. Also, there would not be enough of the molten resin.
Prior Art 1 can be implemented as a mirror or similar optical element needing a single mirror surface, because it roughens the mold surface facing the transfer surface. However, Prior Art 1 cannot produce a lens, prism, or similar optical element, because the number and positions of the mirror surfaces are limited. In addition, the relationship in adhering force is inverted and causes the mirror surface of the molded article or product to sink, depending on the material constituting the transfer surface and roughened surface and the kind of resin used.
Prior Art 2 increases the cost of the mold assembly due to the porous member and provides more sophisticated control over the configuration of the porous member. Specifically, if the effect of the porous member is excessive, it not only admits the molten molding material thereinto, by also obstructs the parting of the molded article or product from the mold assembly. This is particularly true when the porous portion of the porous member extends inwardly over the wall of the mold assembly. Further, because the compressed gas is fed to the non-transfer surface of the molded article or product via the porous member during the previously stated interval, a pressure difference is maintained between the non-transfer surface and the transfer surface of the molten resin during cooling. As a result, the internal strain remains in the resulting molded article or product after the opening of the mold assembly. The residual pressure not only lowers the accuracy of the transfer surface, but also causes the entire molded article or product to deform.
Prior Art 3 generates a pressure difference between the transfer surface and the non-transfer surface of the resin during the interval mentioned earlier. This also brings about the problem stated above in relation to Prior Art 2. Prior Art 4 maintains the transfer surface of the mold assembly at high temperature and heats the transfer surface side of the resin to a high temperature during the previously mentioned interval. This is also undesirable in the above-described respect.
It is therefore an object of the present invention to provide a plastic molded article or product, wherein only a desired portion thereof sinks, wherein a mirror surface is provided in another desired portion thereof, and wherein a method and an apparatus for producing the same by injection molding are further provided.
It is another object of the present invention to provide an inexpensive and least deformable plastic molded article or product, wherein only a non-transfer surface thereof sinks so that the molded article or product has a highly accurate transfer surface.
In accordance with the present invention, in a molded article or product produced by an injection mold assembly having a pair of molds including an inner mold surface forming a cavity having a preselected volume, at least one transfer surface for transferring a mirror surface to the molded article or product from the inner mold surface, and a gate or sprue for filling the cavity with a molten molding material by injection, and by injecting the molten molding material into the cavity via the gate or sprue and then cooling the molten molding material, the injection mold assembly includes at least one vent hole having a preselected opening area, and at least one bore communicating with the vent hole for applying a preselected air pressure to the molded article or product. A step portion is formed on the inner mold surface between the vent hole and the transfer surface.
Also, in accordance with the present invention, in an injection molding method for producing a molded article or product by using a mold assembly having a pair of molds including an inner mold surface forming a cavity having a preselected volume, at least one transfer surface for transferring a mirror surface to the molded article or product from the inner mold surface, and a gate or sprue for filling the cavity with a molten molding material by injection, and by injecting the molten molding material into the cavity via the gate or sprue and then cooling the molten molding material, the inner mold surface is formed with, outside of the transfer surface, with at least one vent hole having a preselected opening area and at least one bore communicating with the vent hole for applying a preselected air pressure to the molten molding material. The air pressure is continuously generated via the vent hole even after the pressure of the molten molding material in the cavity has dropped to zero.
Further, in accordance with the present invention, a mold assembly has a pair of molds including an inner mold surface forming a cavity having a preselected volume, at least one transfer surface for transferring a mirror surface to a molded article or product from the inner mold surface of the mold assembly, and a gate or sprue for filling the cavity with a molten molding material by injection, and injects the molten molding material into the cavity via the gate or sprue and then cools the molten molding material. The inner mold surface is formed with, outside of the transfer surface, at least one vent hole having a preselected opening area and at least one bore communicating with the vent hole for applying a preselected air pressure to the molten molding material, and at least one exhaust hole located at a position adjoining the vent hole, but not facing the transfer surface.
Moreover, in accordance with the present invention, a method of producing a plastic molded article or product begins with the step of preparing a mold assembly including at least one transfer surface and at least one non-transfer surface formed on a surface other than the transfer surface. The transfer surface and non-transfer surface forms at least one cavity. Molten resin, heated to a temperature above a softening point thereof, is injected into the cavity. A resin pressure is caused to act on the transfer surface to thereby cause the molten resin to adhere to the transfer surface, and then the molten resin is cooled to a temperature below the softening point. The mold assembly is opened in order to allow the resulting molded article or product to be taken out. The temperature of at least one non-transfer surface of the resin is lowered below the temperature of the resin on the transfer surface during an interval between the beginning and the end of injection of the molten resin into the cavity.